The invention disclosed relates to a facility for monitoring electric welding operations, in particular those effected on metal box members, employing equipment of the type in which the welding rollers operate in conjuction with a continuous electrode wire looped around guide rollers and fed at a rate matching the surface speed of the welding rollers.
In equipment of the type, the electrode wire creates an external circuit in parallel to the work located between the rollers, which is in receipt of a welding current deriving from the pulsed output voltage supplied by a converter and flowing substantially constant, at least within each half-period.
According to the invention, welding equipment of this general type is provided with a monitoring facility incorporating a microprocessor system that serves to detect defects in the single weldments, hence in the entire metal box, and enable subsequent rejection of sub-standard items.
In the art field of resistance welding, as related in particular to the joining of metal box members, use is normally made of equipment designed to supply a non-constant welding current to the rollers; sine wave welding current, for example, might be adopted.
A first method of monitoring the weld produced along the joined edges of metal box members involves attaching a piezoelectric device to either one of the welding rollers (conventionally the top roller), which picks up any oscillation to which the roller itself may become subject during welding. Clearly, the roller may deviate to a greater or lesser degree from its initial level of contact with the work, according to the temperature generated in and around the work.
By prescribing maximum and minimum degrees of shift for the welding roller fitted with the piezoelectric device in relation to the remaining roller, within which the weld can be considered free of defects, one is provided with a first method of monitoring the accuracy of a finished weld in the face of variations in welding current. Needless to say, a method of this type is bound to be influenced by mechanical factors during operation of the welding equipment, and it is therefore a strictly indirect relationship that exists between oscillation of the roller associated with the piezoelectric device, and quality of the weld.
In a second method, likewise adopted for equipment of the type supplying non-constant current to the welding rollers, it is the welding current itself that is monitored. By measuring the welding current, and adopting set parameters that correspond to the limits of intensity within which it can be safely assumed that the weld is good, it becomes possible to establish whether or not the weld falls within prescribed margins of quality.
In both the first and the second method outlined above, use is made of monitoring facilities designed to operate to prescribed parametric limits within which oscillation, of the welding roller associated with the piezoelectric device in the first instance, of the welding current in the second instance, must register. Such monitoring facilities will generally incorporate microprocessor systems that are able to detect and recognize welding defects in the single boxes, and accordingly, to relay control signals to a knock-out device by which any sub-standard boxes are ejected.
In the second method mentioned above, in particular, measurement of the oscillations in welding current (the welding voltage remains substantially constant) gives rise to the problem that high speed operation of the welding equipment necessarily dictates working with high welding current frequencies. It therefore happens that frequencies of this order produce inductive reactance much higher than the resistance of the welding circuit, which in effect is the parameter used for monitoring purposes. This means that the discernable variations in welding current are small, given that its essential component is substantially inductive.
This much said, it should also be noted that the constant current operating characteristic is being increasingly adopted for resistance welding; in this instance, "constant" signifies that pulsed welding current remains constant on average, at least within each half-period.
Equipment operating in this fashion is favored by virtue of its appreciable advantages over the other types mentioned above.
With constant current type equipment, commonplace quality problems (breaks occurring in the weld) can be almost totally eliminated, and operating tempos can be obtained that were hitherto impossible. Using equipment of this kind however, which operates at substantially constant current, no possibility exists of using the monitoring facilities described above.
Accordingly, the object of the invention is to provide a monitoring facility for electric welding equipment, in particular as employed for metal box manufacture, of the type wherein welding current is constant to all practical intents and purposes.
Another object of the invention is to eliminate those drawbacks which beset the monitoring systems described above, and to afford a type of control directly and closely linked to the welding process, inasmuch as the parameter adopted is the resistance created between the welding rollers during welding. According to the invention, it is the variation in such resistance that constitutes the fundamental parameter in verifying the quality of the weld.